The spontaneous generation of life on Earth
If we do not accept the hypothesis that life may have arrived on Earth coming from space, it is necessary to admit the possibility of spontaneous generation, with a difference, however, compared to the past. In fact, before Pasteur it was thought that the generative process took place quickly and continuously, while the modern theory of the spontaneous origin of life suggests that the process took place slowly and only once.
There are two hypotheses about the new spontaneous generation: the autotrophic hypothese and the heterotrophic hypothese. According to the first of these hypotheses, the first living being would have been an autotroph, ie an organism similar to current green plants, capable of synthesizing organic substances using inorganic substances through a complex series of chemical reactions, which takes the name of "chlorophyll photosynthesis"; in the second hypothesis, the first living being would have been a heterotrophic, that is, an organism that is not able to produce food for itself, but has to take it from other living organisms.
The autotrophic hypothesis arises from the observation that animals (heterotrophs) need plants (autotrophs) to live, while plants need no one to live. To this hypothesis, however, some flawless criticisms are raised from the logical point of view that make it difficult to set up an experimental program that is actually practicable. How is it possible, one wonders, that autotrophs, which are organisms made up of well-organized organic matter, appeared before the substances they themselves produce? Furthermore, autotrophs, in some respects, are more complex than heterotrophs and therefore to suppose that they appeared first would contradict the evolutionary theory, according to which the simpler life forms precede the more complex ones, and do not follow them.
Scientists, therefore considering the appearance of complex organisms in an environment made up of simple molecular forms very unlikely, have turned to the other hypothesis by starting research aimed at demonstrating the possibility of a spontaneous transition from the simple to the complex, that is, from the inorganic world of small molecules to the organic one of large molecules and then still further up to the finely coordinated structures present in living beings. The first ideas in this regard were advanced, in the late 1920s, by the Anglo-Indian biologist John Burdon Sanderson Haldane (1892-1964). He started from the observation that the primitive Earth must have had very different characteristics from the present one. In it, to begin with, there was no life, while in the present one there is life.
The same concept had already been formulated long before by Darwin who in a letter to a friend expressed himself as follows:
it is often said that all the conditions for the first production of a living being are now present, which could ever have been present. But if (and oh what a big if) we could conceive in some warm little pond with all sort of ammonia and phosphoric salts,—light, heat, electricity present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present such matter would be instantly devoured, or absorbed, which would not have been the case before living creatures were formed
The same ideas of Haldane had previously been advanced by a Soviet researcher named Aleksandr Ivanovic Oparin who published them in 1924 in a booklet which was translated into English only in 1937. Oparin and Haldane were both of atheist training and therefore, far from conditioning and prejudices of a religious nature, they approached the problem relating to the origin of life from a purely materialistic point of view. There was only one substantial difference between Haldane's hypothesis and Oparin's and it concerned the primitive atmosphere which according to the Russian scientist must have been rich in hydrogen while, for the English one, it was rich in carbon dioxide. And it is precisely from this data that we will start to have experimental confirmation of the new ideas.
The ideas of Haldane and Oparin were not accepted willingly by believers who tried to demonstrate that life could not have been born through the fortuitous join of atoms, and for this reason it could only be the fruit of divine intervention. For example, proteins - they said - are very complex molecules and to pretend that they can be formed through the chance joining of the atoms that constitute them is without logic. Indeed, it is extremely unlikely that the fundamental compounds of living matter are the result of the random collision of hydrogen, oxygen, carbon and nitrogen atoms, to name only the fundamental ones. The thing then becomes absurd if we consider that the first complex molecules would have had less than a billion years to organize themselves starting from simple elements, and a billion years is a very limited time for this kind of operation. This type of reasoning, however, is wrong because it imagines that atoms must join together in a completely random way while it is shown that the possible combinations of the simplest constituents of matter are not infinite but limited, and guided by restrictive chemical and physical laws. All these assumptions were promptly confirmed in the laboratory.
THE PRIMORDIAL ATMOSPHERE
The environmental conditions present on Earth before living beings appeared had to be very different from the current conditions and in particular the composition of the atmosphere that enveloped our planet had to be different. The primitive atmosphere was certainly devoid of oxygen, a very reactive gas which is currently being reintegrated by plants as it is consumed because it combines with the other elements. Therefore, when plants did not yet exist, even if oxygen had been present in small traces, coming, for example, from the splitting of water molecules as a result of electrical discharges, this would have immediately reacted with many of the existing elements, oxidizing them.
In the language of chemistry, the terms "oxidizing" and "reducing" mean the ability of atoms to attract or remove electrons when they bind to other atoms to form compounds. This phenomenon occurs every time that atoms combine with each other and especially when they do so with oxygen and hydrogen, which are two very common elements in nature and which form compounds with practically all the others. For example, when any atom binds to oxygen to form a compound, this atom oxidizes because oxygen attracts its electrons to itself. If the same atom binds to hydrogen instead it is reduced, because in this case the hydrogen moves its electron on that atom. Carbon, for example, can bind one or two oxygen atoms to itself: in the first case a compound is formed which is called carbon monoxide (CO) and in the second case carbon dioxide (CO2) is formed and it is said that carbon is found in the most oxidized form.
The same carbon atom can also bind to hydrogen forming the so-called hydrocarbons (composed of only hydrogen and carbon) and if the hydrogen atoms that bind around it are four, that is the maximum allowed to the single atom, it is said that carbon it is found in the smallest possible form, corresponding to the methane molecule (CH4). When hydrogen and oxygen bind together, the water molecule (H2O) is formed in which the respective reducing and oxidizing characteristics of the two elements cancel each other out, forming a neutral compound from the point of view of these properties.
Now let's see how the atmosphere on the early Earth was formed. Geophysicists have calculated that the Sun and the planets surrounding our planet were formed approximately 5 billion years ago following the explosion of a supernova, that is, a very large star that, before disintegrating, had been able to synthesize at its internal many heavy elements starting from hydrogen and helium. Therefore, the Earth was originally an incandescent sphere formed mainly of hydrogen and helium, but also of heavy elements such as carbon, nitrogen, oxygen, iron and silicon that had been projected into space by the explosion of the supernova.
Then the Earth cooled down, assuming a conformation quite close to the current one and the lighter gases partly reacted with the heavier elements and partly dispersed into space.
In particular, almost all the helium, which is a light gas and not at all reactive, was removed, while a part of the hydrogen, the lightest element of all, combined with other elements to form simple hydrogenated compounds such as methane (CH4) , ammonia (NH3), hydrogen sulphide (H2S) and water (H2O).
Once the "cosmic removing" of the lighter elements was finished, the heavier ones remained in place, which began to differentiate by the action of gravity in a central "core" formed almost exclusively of iron and nickel, in an overlying "mantle" made up of oxides of elements heavy and in a superficial "crust" made of silicates of light elements such as aluminum, potassium and sodium. During the formation and consolidation of the crust, many gases and easily volatile substances from inside the planet were released through the cracks in it, which formed what is considered the primordial atmosphere of the Earth.
An indirect proof of the composition of this primordial atmosphere is given by the examination of the gaseous mixtures emitted nowadays by volcanoes and by sulfataras, whose composition is very close to that obtained theoretically. Another proof was provided by the probes that reached the outermost planets of the solar system where they found the existence of an atmosphere rich in hydrogenated compounds. The same conclusions are finally reached with the analysis of meteorites, in which all substances are present in highly reduced form.
There is no reliable evidence on the composition of the primitive atmosphere, but scientists are certain of one thing: in that atmosphere there was no free oxygen (O2), not even in very modest quantities. Now, however, if in those ancient times there was no oxygen, there should not even be ozone (which is a compound made up of three oxygen atoms, instead of two) and therefore the ultraviolet light of the Sun, which currently it is blocked by the presence of a thick layer of this gas in the upper atmosphere, could reach the surface of the Earth in much greater quantities than today and contribute, with its energy, to the formation of primitive chemical compounds.
The ultraviolet radiation of the Sun will certainly have been an important energy source for the synthesis of organic compounds, but not the only one, also because that type of radiation, as well as forming them, decomposes many organic molecules. The other electromagnetic radiations coming from the Sun and in particular the visible component of them, had no efficacy for the primordial syntheses of organic compounds, while they will be decisive in the subsequent stages of the development of life. Very important, however, for the primordial organic syntheses, was the energy deriving from electrical discharges.
THE MILLER EXPERIMENT
In 1952 a young American researcher, Stanley Lloyd Miller, fresh of college, was commissioned by his professor, Harold Clayton Urey, Nobel laureate in chemistry in 1934, to carry out an experiment.
The necessary equipment, built on purpose, was quite simple and consisted of two glass bowls filled respectively with water kept at high temperature and a mixture of hydrogen (H2), ammonia (NH3) and methane (CH4), that together with water vapor (H2O), they were believed to be the main constituents of the gaseous envelope that surrounded the Earth more than four billion years ago. The hot water, which the researchers intended to represent the ancestral ocean, produced steam which was conveyed through a tube into the vessel which contained the gases of the primitive atmosphere. Inside that vessel 60.000 volt electrical discharges were generated which should have reproduced the presumably frequent and intense thunderstorm phenomena at the beginning of our planet's history.
After a week of continuous treatment, the content of the bowl full of water was analyzed, which in the meantime had changed color, becoming red-orange, and with surprise, some amino acids, that are the precursors of life, were discovered inside it, together with several other compounds like proteins which, as everyone knows, are the main constituents of living organisms.
Miller was not the first chemist to synthesize amino acids that had actually already been obtained in the laboratory by other means, but he was the first to demonstrate that starting from very simple compounds, which were supposed to be present in the primitive atmosphere of the Earth, it was possible to obtain particular complex molecules, that is precisely those molecules that are the basis of the organic compounds that characterize living beings. And all this without resorting to particular artifices or exceptional energy sources.
Miller's experiment was followed by other scientists with different starting gas mixtures, but always containing the fundamental elements of living organisms, namely carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S) and phosphorus (P). The sources of energy used were of various types. Ultraviolet rays, X rays, electron flows or simply high temperatures were used, thanks to which various compounds typical of living organisms were obtained such as carbohydrates, lipids, amino acids and even nucleotides, ie the constituents of DNA and RNA. Molecules other than those typical of present living matter were never found.
All these experiments have shown, unequivocally, that the biological precursors of living organisms could have been formed in the primitive atmosphere, if this had been a reducing atmosphere, through normal chemical processes of synthesis. These simple organic compounds in a second time would have fallen to the ground carried by the rains and then conveyed into the sea where, at a later time, they could possibly enrich and integrate. Of course, producing amino acids in a test tube does not mean creating a living being: a cell is in fact as much more complex structure than an amino acid as a man compared to a cell.
In truth, not everything is clear: for example, experiments have shown that complex prebiotic molecules are formed more easily at high temperatures than at low temperatures. Furthermore, the composition of living matter seems to be closer to that of stars than to that of the Earth, where compounds based on silicon, oxygen, calcium, sodium and iron predominate. Finally, to further complicate matters, there is the recent discovery of some organic compounds, even quite complex, inside galactic clouds.
Well this set of observations led Fred Hoyle and his Indian collaborator C. Wickramasinge to believe that life could have formed among the stars instead of on Earth; in a sense, the old theory of panspermia has been revived.
Fred Hoyle is a well-known astronomer, the one who proposed the famous "Stationary Universe Model", that is the model according to which the Universe would be infinite and eternal and in which new matter would be continually created at the same rate with which the expansion under way would be rarefying the existing one. The theory of panspermia, advanced by Hoyle in a new light, would therefore be consistent with his model of the Universe. In a Universe that had an origin, as we have already pointed out, supposing that life came to Earth from space would not solve the problem of its appearance, but would simply move it to another planet. In an eternal Universe, however, even life could be eternal.
In reality, many organic molecules have been discovered in the interstellar clouds and in the tails of comets, some of which are also of the type that underlies vital phenomena, but it is not yet clear how these molecules were formed. However, the discovery of organic molecules in the galactic clouds does not authorize us to assume the source of the materials essential for the development of life and life itself from space. For Hoyle, however, the observations of molecular clouds are sufficient to believe that the first living organisms, those that presumably were able to live without oxygen and at temperatures close to absolute zero, were formed in space and not on Earth. According to the English astronomer, comets not only brought the first germs of life to Earth.
